Possible your onboard nic is not recognized, try the link below
http://wiki.linuxmce.org/index.php/Realtek_8168You can check if a driver is loaded with the nic, using lspci -v|more and look for you nic card see eg below
04:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 02)
Subsystem: ZOTAC International (MCO) Ltd. Device 8168
Flags: bus master, fast devsel, latency 0, IRQ 218
I/O ports at be00 [size=256]
Memory at fd8ff000 (64-bit, non-prefetchable) [size=4K]
Memory at fd7f0000 (64-bit, prefetchable) [size=64K]
[virtual] Expansion ROM at fd700000 [disabled] [size=128K]
Capabilities: <access denied>
Kernel driver in use: r8169
Kernel modules: r8169
05:00.0 Ethernet controller: Intel Corporation 82572EI Gigabit Ethernet Controller (Copper) (rev 06)
Subsystem: Intel Corporation Device 1082
Flags: bus master, fast devsel, latency 0, IRQ 216
Memory at fd6e0000 (32-bit, non-prefetchable) [size=128K]
Memory at fd6c0000 (32-bit, non-prefetchable) [size=128K]
I/O ports at af00 [size=32]
[virtual] Expansion ROM at fd500000 [disabled] [size=128K]
Capabilities: <access denied>
Kernel driver in use: e1000e
Kernel modules: e1000e